Municipal wastewater having more or less large amounts of industrial wastewaters is purified in a plant consisting of a primary clarifier, an aeration tank and a secondary clarifier. The wastewater, which, depending on the time of year, has a temperature from 7 to 25° C. and a nitrogen concentration from about 30 to 70 mg/l, after a first mechanical cleaning, which consists of a screen system and a primary clarifier, is introduced into an aeration tank.
In the aeration tank the actual biological purification of the wastewater takes place. Here, the nitrogen compounds which are bound in the wastewater, such as ammonium (NH4), nitrite (NO2) and nitrate (NO3), are converted by nitrification/denitrification to elemental nitrogen (N2) which is released in the gaseous state into the ambient air as a harmless end product. During the nitrification, ammonium is oxidized by oxygen via the intermediate nitrite to form nitrate. In the subsequent denitrification, the nitrate is reduced in a first reduction step to nitrite, and in a second reduction step, to nitrogen.
The biological nitrification/denitrification has the disadvantage of a high oxygen demand and therefore high energy consumption. In addition, organic carbon is consumed in the denitrification which is disadvantageous for the further purification process and the sludge properties.
After the biological purification of the wastewater in the aeration tank, the wastewater-sludge mixture is passed into the secondary clarifier of the plant in which the water is separated from the sludge, wherein the separated water is removed from the secondary clarifier and disposed of and the sludge is in part returned as return sludge to the aeration tank and in part fed as surplus sludge to a digestion tank. In the digestion tank, or during the transport to the digestion tank, the sludge is heated to a temperature of about 40° C. During the sludge digestion, the organic constituents of the surplus sludge from the secondary clarifier and of the sludge withdrawn from the wastewater in the primary clarifier are converted into gas (methane). The nitrogen present remains in the sludge in which there is now a high nitrogen concentration of typically 500 to 2000 mg/l. This high-nitrogen sludge, after the sludge digestion in the digestion vessel, is fed to an appliance for sludge dewatering, for example a centrifuge. The aqueous phase, after the sludge dewatering, contains the nitrogen and has a temperature of about 25 to 39° C. The warm, high-nitrogen sludge water is then fed to a deammonifying tank, while the sludge that is separated off from the sludge water is disposed of.
In the deammonifying tank, the nitrogen compounds (NH4, organic nitrogen) that are present in the sludge water are converted by deammonification into elemental nitrogen (N2) which is outgassed to the ambient air. The surplus sludge formed in the deammonification is then fed to the sludge treatment.
Deammonification is an efficient method for biological elimination of nitrogen, in particular for the purification of wastewaters having high ammonium concentrations. In the biological deammonification with suspended biomass, two bacterial groups participate, firstly the aerobically ammonium-oxidizing bacteria (AOB) which convert ammonium to nitrite, and secondly the anaerobically ammonium-oxidizing and elemental nitrogen-producing bacteria (ANAMMOX), in particular Planctomycetes, which carry out this step with the aid of the previously produced nitrite.
The aerobically ammonium-oxidizing bacteria (AOB) produce 10 times more new bacterial mass, based on the material conversion rate, than the anaerobically ammonium-oxidizing bacteria (ANAMMOX). The residence time of the sludge in the sludge system must therefore be at least long enough that the slow-growing anaerobically ammonium-oxidizing bacteria (ANAMMOX) can accumulate.
Compared with the nitrification/denitrification, during the deammonification, only half of the oxygen is required, or the energy consumption for nitrogen elimination is halved. The deammonification is an autotrophic process in which no organic carbon is required. Therefore, the remaining purification process is more stable.
Methods for single-stage or two-stage biological deammonification are already known from WO 2007/033393 A1, EP 0 327 184 B1 and WO 00/05176 A1.
In particular, the substantially longer generation times of the anaerobically ammonium-oxidizing bacteria (ANAMMOX) have proved to be disadvantageous in the deammonification, which generation times are longer by the factor 10 to 15 than those of the aerobically ammonium-oxidizing bacteria (AOB). As a result, a stable system can only form when the residence time of the sludge or of the bacteria in the tank is sufficiently high. This in turn necessitates large reaction volumes and correspondingly constructed tanks.
In addition, a sufficiently high wastewater temperature (>25° C.) is fundamental for the growth of the anaerobically ammonium-oxidizing bacteria (ANAMMOX) on an industrial scale. However, heating of the wastewater is highly demanding in terms of energy, for which reason the methods described cannot be used or carried out economically with wastewaters at low temperatures.
In addition, the presence of those bacterial groups that convert the nitrite formed into nitrate under aerobic conditions (“nitrite-oxidizing bacteria,” or NOB) proves to be disadvantageous. This group of bacteria has generation times shorter by the factor 10 compared with the anaerobically ammonium-oxidizing bacteria (ANAMMOX).
For the said reasons, the application of deammonification is restricted to warm wastewater streams which simultaneously have a high nitrogen concentration. Application of deammonification in the case of cold wastewaters having low nitrogen concentrations would necessitate very high reaction volumes that are not economically expedient. Conventional nitrifying plants already require a tank volume that typically must ensure a sludge age of 15 to 20 days. For application of deammonification, these tank volumes would have to be increased further by the factor 10 to 15.
Furthermore, EP 0 503 546 B1 discloses a method for purifying wastewater by nitrification/denitrification, in which in a collection container, a metabolically active nitrifying bacteria population is cultured, from which biomass is transferred continuously or at intervals into a biological purification step, where a post-loaded material stream having a high nitrogen loading is passed into the collecting container.